Richard Thomas, the son of retired Welsh farmer Llywelyn Thomas, says he was beaten to death by two burglars despite presenting no threat to them at all.
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Lukasz Pawlowski cut his throat with a kitchen knife as he was due to be sentenced in Havefordwest. He has been remanded in custody.
The peer will attack Prime Minister Theresa May for failing to represent the 48% who voted in favour of Remain.
Paul Flynn, MP for Newport West, will open the debate in Westminster Hall.